Early and mid-Holocene climate in the tropical Pacific: seasonal cycle and interannual variability induced by insolation changes
1State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics (LASG), Institute of Atmospheric Physics (IAP), Beijing, China
2Laboratoire des Sciences du Climat et de l'Environnement (LSCE/IPSL), Unité mixte CEA-CNRS-UVSQ – UMR8212, Orme des Merisiers, Gif sur Yvette, France
3Graduate University of Chinese Academy of Sciences, Beijing, China
Abstract. Using a coupled atmosphere-ocean model we analyze the responses of the mean climate and interannual variations in the tropical Pacific to the changes in insolation during the early and middle Holocene, for which only the variations of Earth's orbital configuration are considered. Comparison of the early and mid-Holocene with pre-industrial climate shows that both the mean climate and the characteristics of the interannual variability are altered by the changes in insolation. In particular, there is a decrease of the annual mean SST, which is characterized by a "U" shape across the tropical Pacific. The changes of the SST seasonal cycle are consistent with the changes in insolation, with the SST amplitudes weakening in the tropics. However, the larger changes in seasonality are found in the eastern Pacific, where thermodynamics and dynamical processes strengthen the SST response. The cloud radiative forcing largely reduces the shortwave radiation in the western tropical Pacific in winter causing a zonally asymmetric heat flux response. Simulations also show that ENSO strengthens across the Holocene, as suggested by coral data or lake sediments. The role of the obliquity is examined by a sensitivity experiment and we find that the obliquity change affects the seasonal displacement of ITCZ related to strength of SST meridional gradients. However, the obliquity change has little effect on SST seasonal cycle and interannual variability in eastern tropical Pacific. The precession of the orbital parameter is more important in effecting the tropical climate.